# Integer basic proofs

• Jul 20th 2007, 11:06 AM
Dili
Integer basic proofs
Hello,

Can anyone help me with any of the foolowing proofs

For any integers a,b,c,d
1. a | 0, 1 | a, a | a.
2. a | 1 if and only if a=+/-1.
3. If a | b and c | d, then ac | bd.
4. If a | b and b | c, then a | c.
5. a | b and b | a if and only if a=+/-b.
6. If a | b and b is not zero, then |a| < |b|.
7. a+b is integer, a.b is integer
Thank you
• Jul 20th 2007, 12:19 PM
ThePerfectHacker
Quote:

Originally Posted by Dili
Hello,

Can anyone help me with any of the foolowing proofs

For any integers a,b,c,d
1. a | 0, 1 | a, a | a.
2. a | 1 if and only if a=+/-1.
3. If a | b and c | d, then ac | bd.
4. If a | b and b | c, then a | c.
5. a | b and b | a if and only if a=+/-b.
6. If a | b and b is not zero, then |a| < |b|.
7. a+b is integer, a.b is integer
Thank you

Let me do the first ones.

Definition: Let $a,b\in \mathbb{Z}$ then we define $a|b$ iff $b=ac \mbox{ for some }c\in \mathbb{Z}$.

Theorem: Let $a\in \mathbb{Z}$ then $a|0$.

Proof: We need to show $0=ac$ for some $c\in \mathbb{Z}$. So choose $c=0$.Q>E>D>

Theorem: Let $a\in \mathbb{Z}$ then $1|a$.

Proof: We need to show $a=1c$ for some $c\in \mathbb{Z}$. So choose $c=a$.Q.E.D.

Theorem: Let $a\in \mathbb{Z}$ then $a|a$.

Proof: We need to show $a=ac$ for some $c\in \mathbb{Z}$. So chose $c=1$.Q.E.D.

Theorem: Let $a\in \mathbb{Z}$ and $a|1$ then $a=\pm 1$.

Proof: We need to find $1=ac$ for some $c\in \mathbb{Z}$. If $|a|\geq 2$ then $|ac|>1$ which is impossible. If $a=0$ it is impossible. So the only possible case is $|a|=1$.Q.E.D.
• Jul 21st 2007, 03:05 AM
Dili
Thank you.
I shall welcome any other proofs to the others especially the 7th one
ie. a+b and a.b are integers
• Jul 21st 2007, 06:44 PM
ThePerfectHacker
Quote:

Originally Posted by Dili
ie. a+b and a.b are integers

That has nothing to do with number theory. That is more of a Set Theory question. So I have no idea what you want with that one.
• Jul 22nd 2007, 08:25 AM
tukeywilliams
For the seventh one, isnt that the very definition of a field? Really I think its a closure axiom, so you dont need to prove it.
• Jul 22nd 2007, 09:31 AM
ThePerfectHacker
Quote:

Originally Posted by tukeywilliams
For the seventh one, isnt that the very definition of a field?

The integers are not a field.

Quote:

Really I think its a closure axiom, so you dont need to prove it.
That is not how this axiom thing works, i.e. you cannot simply say it is an axiom. Before you state something is closed, you need to actually show it is closed. You cannot just say that is an axiom, that makes no sense.

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This question (7) should just be avoided. It has nothing to do with number theory.